1. Field of the Invention
The present invention relates to a flow sensor installed in a fluid path for measuring a physical property of a fluid, i.e., a gas or liquid, flowing through the fluid path, and in particular, the present invention may be applied to a thermal-type flow sensor formed on a substrate which is applicable to the control and measurement of a flowing velocity or flow volume of a gas flow or liquid flow.
2. Discussion of the Background
Various types of sensors for measuring various physical properties, e.g., a flowing velocity or a flow volume, of a gas flow or a liquid flow are known, including one in which a thermal-type flow sensor is formed on a substrate.
FIG. 1 shows an example of a background thermal-type gas flow sensor formed on a substrate as disclosed in Japanese Laid-Open Patent Application (KOKAI) 5-157603.
As shown in FIG. 1, a bridge-like thin film sensor 5 is suspended over a hollow 3 of a square semiconductor substrate 1. An upstream-side temperature sensing portion 9 and a downstream-side temperature sensing portion 11 each constructed with an electric resistor are formed on the bridge-like thin film sensor 5. A heater constructed with an electric resistor is formed between these sensing portions 9, 11 on the bridge-like thin film sensor 5. In a case that a gas does not flow in the flow direction A, the upstream-side sensing portion 9 is heated the same as the downstream-side sensing portion 11 by the heater 7. However, in a case that a gas flows in the direction A, the upstream-side sensing portion 9 is heated more than the downstream-side sensing portion 11 by the warm gas heated by the heater 7. As resistance values of these sensing portions 9, 11 vary due to the temperature variation, a flowing velocity or flowing volume of the gas can be measured by the resistance value difference between these sensing portions 9, 11.
FIG. 2 is sectional side view showing a tube 15 as a fluid path and the substrate 1 of the thermal-type gas flow sensor installed in the tube 15.
The substrate 1 of the flow sensor is installed on an inner wall 17 of the tube 15. A wire 13 outputs measurement signals from sensing portions 9, 11.
According to an experiment of the sensor as shown in FIG. 2 using a circuit as shown in FIG. 3 conducted by the applicants of the present invention, a relation of power of an output signal V from the sensing circuit and a flow rate Q is shown as in FIG. 4. As shown in FIG. 4, the power of output signal V becomes unstable on a fast flow. This unstableness is understood to be caused by a turbulence of the flow. As can be seen from the above description, it is very important to reduce a turbulence of the flow.